Picuris Mountains

By Ronan DENNIEL and Nicolas AUBERT

Tuesday 23 may 2006 :

During the first day of this trip we went to the Picuris mountains. Theses mountains are located on the north of the Rio Grande Rift.

The Picuris mountains are mostlly composed with metamorphic and igneous rocks.

The first outcrop is an old mine called “Harding mine”. This outcrop is about ½ miles long. The pegmatite is 1,4 Byr.

There is an intrusive contact between the pegmatite and the amphibolites which are 1,7 Byr The contact is shown on Figure 1.

There is some Zircon in these rocks: it could mean there is a lot of metamorphism and igneous rocks beside the pegmatite.

Figure 2. Map of the pegmatite mine kindly provided by Aubri A. Jenson

Wednesday 24 and Thursday 25 may 2006:

mapping in “Cerro de la Marquenas” in the Picuris mountains.

We started the mapping project in the Picuris mountains on this beautiful Wednesday morning.

The formations we can observe are metamorphic Precambrian rocks. The first one is the Marquenas formation which is composed of quartzite and metaconglomerates. Some cross stratifications show us that this formation is overturned. Then, we enter in the younger formation of Piedre lumbre formation which is Silver gray , fin-grained, with graphitic garnet-biotite schist. the contact between the Marquenas formation and the Piedre Lumbre formation is not really easy to see on the field, but it seems to be a faulted contact which leads to this unconformity. This is most probably a reverse fault since older rocks of the Marquenas Formation are placed over younger rocks of the Hondo Group. While going down the hill (from south to north), we enter the older formations of Pilar formation (composed of dark gray to black phyllite with extremely fined grain). There are some beautiful garnets and other metamorphic minerals. In the rattle snake gorge (no rattle snake but some crows and Nico saw a bear) we walk trough the Rinconada formation, where we see cooper deposits. The Rinconada formation is composed of interbedded quartzite and schist of diverse colors and composition. This rinconada formation is 700 meters thick, it is the most thick formationof the precambrian. Then in the canyon we find older Precambrian formation: the Ortega formation, which is composed of diverse massive to cross-bedded quartzite, andalousite, kyanite, and some garnet locally.

This took about too days of hard mapping with about a hundred degrees Fahrenheit, we had good discussions about the faulted contact between the Marquenas formation and the Piedre Lumbre formation. But the final idea was a reverse fault dipping toward the south.

We conclude that the Cerro de la Marquenas was composed by an anticline of Precambrian rocks with the Ortega formation in its heart. The unconformity between the Piedre Lumbre and the Marquenas formations was supposed to be a reverse fault even if we didn’t saw the contact.